Add support for index aliases.

[youssef3wi]

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Closes #2599

[sothawo]

Sorry it took me a while to get to this.

I don't think that AliasCoordinates should be added to IndexCoordinates. IndexCoordinates is used throughout the library wherever an index name or names are passed. Adding the alias information might cause the user to think he might add some alias info to other functions where this is definitely not. I think that the alias information should be passed to the IndicesTemplate.create() and ReactiveIndicesTemplate.create() functions like it's done with mapping and settings. This is something only relevant during index creation.

Also I am not sure about reusing the @Query annotation. In Spring Data context this normally only exists on repository methods. The documentation for @Query says that the query can contain parameters like ?0, this of course is not valis in the index creation process. I'd rather suggest to creat a new @Filter annotation that is only used in the index creation context.

[sothawo]

Sorry, have been pretty busy (and sick in between), just a first remark, had no time to look further

[sothawo]

nice work, thank you.

Some minor things. And the reactive integration test is missing